using System;
using Sandbox;

namespace Diamonds;

public sealed partial class DiamondPanel
{
	// Used only by editor comparisons; never exposed as a gameplay setting.
	static bool drawingReferenceStrokes = false;

	// Small mitered strips stay on Painter's inline polygon path instead of allocating
	// a stroke primitive tree. This is restricted to the simple gem/fragment geometry.
	static void DrawGemStroke( Painter p, ReadOnlySpan<Vector2> points, Color color, float width, bool closed )
	{
		if ( drawingReferenceStrokes || (!closed && points.Length > 4) )
		{
			p.Fill = Fill.None;
			p.Stroke = Stroke.Solid( color, width );
			if ( closed ) p.Polygon( points ); else p.Line( points );
			return;
		}
		Span<Vector2> offsets = stackalloc Vector2[points.Length];
		for ( int i = 0; i < points.Length; i++ )
		{
			var incoming = (points[i] - points[(i + points.Length - 1) % points.Length]).Normal;
			var outgoing = (points[(i + 1) % points.Length] - points[i]).Normal;
			if ( !closed && i == 0 ) incoming = outgoing;
			if ( !closed && i == points.Length - 1 ) outgoing = incoming;
			var n0 = new Vector2( -incoming.y, incoming.x );
			var n1 = new Vector2( -outgoing.y, outgoing.x );
			float denominator = 1 + Vector2.Dot( n0, n1 );
			var miter = denominator > 0.00001f ? (n0 + n1) / denominator : n0;
			// Match the default four-width miter limit; extreme shard tips use the original path.
			if ( denominator <= 0.00001f || miter.Length > 4 )
			{
				p.Fill = Fill.None; p.Stroke = Stroke.Solid( color, width );
				if ( closed ) p.Polygon( points ); else p.Line( points );
				return;
			}
			offsets[i] = miter * (width * 0.5f);
		}
		p.Stroke = Stroke.None;
		p.Fill = color;
		if ( !closed )
		{
			// A single polygon keeps crack joins seamless. Longer cracks retain Painter's
			// stroke path rather than splitting them across independently antialiased edges.
			Span<Vector2> polygon = stackalloc Vector2[points.Length * 2];
			for ( int i = 0; i < points.Length; i++ )
			{
				polygon[i] = points[i] + offsets[i];
				polygon[polygon.Length - 1 - i] = points[i] - offsets[i];
			}
			p.Polygon( polygon );
			return;
		}
		for ( int i = 0; i < points.Length; i++ )
		{
			int next = (i + 1) % points.Length;
			p.Quad( points[i] + offsets[i], points[next] + offsets[next],
				points[next] - offsets[next], points[i] - offsets[i] );
		}
	}
}